Nuclear magnetic resonance (NMR) relaxation time measurements, although among the most accurate methods to estimate formation porosity, have been considered conventionally as insensitive to the presence of microfractures. Hence, the NMR responses in multiple-porosity systems, which may contain intergranular pores, microfractures, or channel-like inclusions, have not yet been thoroughly investigated. NMR pore-scale simulations using a random-walk algorithm enabled us to quantify the impact of microfractures/channels on NMR measurements and to propose a new concept of fracture-pore diffusional coupling in such heterogeneous systems. We randomly distributed and oriented microfractures (or channels) in 3D pore-scale images of different rock matrices. We then quantified the sensitivity of NMR (spin-spin relaxation time) distribution to the presence of microfractures (or channels) and compared the pore-scale simulation results against a previously published experimental study. The pore-scale simulation results from synthetic rock samples revealed that NMR distribution can be influenced not only by the pore-size distribution but also significantly by fracture-pore diffusional coupling. The intergranular pore size can be underestimated by up to 29%, and the volume fraction of intergranular pores can be underestimated by more than 10%, if the impact of diffusional coupling was not taken into account in interpretation of NMR measurements. Furthermore, we developed a simplified 1D analytical model for fracture-pore diffusional coupling. The analytical solutions of the 1D model were in agreement with the simulation results in the synthetic rock samples, which further demonstrated the existence of fracture-pore coupling in multiple-porosity systems. The developed 1D model enabled real-time evaluation of diffusional coupling effect in the presence of microfractures and complex pore-size distribution. The results were promising for future applications of NMR relaxometry for the assessment of microfracture content, when combined with other conventional well logs.